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<TD borderColor=#6699ff><FONT size=2>您的位置：技术园地>技术讲座-扩频通信</FONT></TD></TR>
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<DIV align=center><FONT size=4><B><SPAN
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color=#000080 size=5>第五讲
直接序列扩频系统(4)</FONT></SPAN></B></FONT></DIV></TD></TR>
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<P class=MsoNormal
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color=#800080><B><FONT size=4><SPAN lang=EN-US>5.4
直扩系统的同步</SPAN></FONT></B></FONT></P>
<P class=MsoNormal
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lang=EN-US style="FONT-SIZE: 12pt; COLOR: #333399">5.4.1<SPAN
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style="FONT-SIZE: 12pt; COLOR: #333399; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">同步原理</SPAN><SPAN
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</SPAN><O:P></O:P></SPAN></P>
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">任何数字通信系统都是离散信号的传输，要求收发两端信号在频率上<BR>相同和相位上一致，才能正确地解调出信息。扩频通信系统也不例外。一个相干扩频数字通信系统，接收端与发送端必须实现信息码元同步、</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码码元和序列同步和射频载频同步。只有实现了这些同步，直扩系统才能正常的工作。可以说没有同步就没有扩频通信系统。</SPAN></P>
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<P class=MsoNormal
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">同步系统是扩频通信的关键技术。在上述几种同步中，信息码元时钟可以和</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码元时钟联系起来，有固定的关系，一个实现了同步，另一个自然也就同步了。对于载频同步来说，主要是针对相于解调的相位同步而言。常见的载频提取和跟踪的方法都可采用，例如用跟踪锁相环来实现载频同步。因此，这里我们只重点讨论</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码码元和序列的同步。</SPAN><SPAN
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lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">一般说来，在发射机和接收机中采用精确的频率源，可以去掉大部分频率和相位的不确定性。但引起不确定性的因素有以下一些：</SPAN><SPAN
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收发信机的距离引起传播的延迟产生的相位差</SPAN><SPAN lang=EN-US
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收发信机相对不稳定性引起的频差</SPAN><SPAN lang=EN-US
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收发信机相对运动引起的多普勒频移；</SPAN><SPAN lang=EN-US
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以及多径传播也会影响中心频率的改变。</SPAN></P>
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">因此，只靠提高频率源的稳定度是不够的，需要采取进一步提高同步速率和精度的方法。</SPAN><SPAN
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">同步系统的作用就是要实现本地产生的</SPAN><SPAN
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